Aerosols and Pressure

Water bath leak testing

During manufacture most filled aerosols are immersed in a hot water bath at 50 ºC for about three minutes This is 30 ºC above normal room temperature The purpose of this test is to check for leaks at well above the temperature that aerosol cans would be exposed to in normal use.

The increased temperature raises the pressure inside the can. Any can that leaks is detected and rejected.
Where the propellant is a liquified flammable gas, any leak has the potential to be dangerous. The highest risk is during transport, when several thousand cans are tightly packed on a truck.

For an ideal gas the Pressure Law will show how the pressure of a fixed volume of gas will change with temperature.

Before doing any calculations we have to be clear about the zero point on this graph. Zero pressure means zero kinetic energy of the gas molecules. This happens at Absolute Zero (zero Kelvin), the lowest possible temperature. Absolute zero is -273 ºC.
To make this easier to see the graph can be drawn like this:

0 ºC is 273 K and so 20 ºC is 293 K.

If T1 is 20 ºC and T2 is 50 ºC then the pressure inside the can will rise from P1 to P2. As the pressure/temperature line is straight (for an ideal gas) it means that the ratio of P to T is constant. The maths only works if the temperature units are Kelvin.

Does the pressure law work for propane and butane vapour?

The answer is no. The graph of pressure against temperature for propane looks like this:

The line stops at -41 ºC because all the propane condenses to liquid below that
temperature. The line is not straight so there is no simple ratio of
pressure to temperature. As the temperature rises, the pressure
increases at a higher rate.

The reason for this is that the evaporation of liquid propane in the can
is creating the pressure. This is the same as the evaporation that
dives a steam engine, but at a lower temperature.

So a vapour over a liquid does not behave like an ideal gas. Engineers and designers need to know this to make sure cans will withstand higher pressures. The water bath test makes sure potentially dangerous cans are rejected.

Comparing propellants

This chart compares two propellants. Nitrogen (red) and a propane/butane mix (blue) This is real data from a test lab.

Nitrogen is an ideal gas and obeys the pressure law. The line is straight.

The propane/butane mix (blue) has a curved line.

You can download the Excel spreadsheet of the data here to make your own charts.